The scientific study of glacial erratics

By Mike Horne FGS*

There has been a long history of the study of glacial erratics: the Hull Geological Society started to publish records in 1894 and revived the tradition in 1987 (Harrison and Horne 1992). This is an example of what now might be called ‘citizen science' as amateur scientists contribute alongside professionals and academics. Most of us can spot and identify erratic pebbles from the beaches of Holderness but are there sources of bias that can affect the data collected and their interpretation?

Size

The original East Riding Boulder Committee started by only recording “boulders”. The modern sedimentary definition of a boulder is a clast that is over 256mm in diameter and most clasts are not perfectly spherical.

There is an obvious bias involved with this. Some rocks occur only as pebbles (4-64mm) such as the very common yellow quartz. Other rocks can only be accurately identified in a larger specimen because they have a large pattern that needs to be seen, such as Shap Granite and Frosterly Marble.

Distribution on the beaches

Once an erratic has been eroded from the diamict in the cliff or beach exposure it will be subject to weathering and abrasion. Tougher rock types such as quartz and igneous rocks will survive longer on the beaches and will become preferentially concentrated compared with weaker rocks, such as chalks, oolites, shales and mudrocks.

The transportation of pieces of the augen gneiss from the sea defences at Mappleton southwards for a considerable distance indicates that longshore drift has a significant effect on the redistribution of pebbles on the beaches of Holderness.

There is also a simple geographical bias in our recording; we can compile long lists of erratics found at places we visit frequently that have good access, such as Mappleton (Horne 2021), and quite short lists from inaccessible sites and temporary exposures.

Identification

The accurate identification of erratics depends on the experience of the geologist. Experience gained by visiting the source areas of the erratics would also be a great advantage. 

We are likely to be inherently biased when we look at the erratics. Our eyes are drawn to the rarer erratics that can tell a story, such as Shap Granite, rather than the more common yellow quartz. We are also unlikely to record in our notebooks the rocks and fossils that we do not recognise.

Transport routes from the assumed source locations

Some erratics stand out because they are easy to identify and can only come from one source, such as Larvikite and Shap Granite. It is then tempting to assume that these erratics travelled directly from their source in a straight line during one glacial episode. However there has been more than one glaciation in the Pleistocene and glaciers do not necessarily travel in a straight line. It is more likely that the glacial erratics have been naturally recycled a number of times before they arrived at the site where we recorded them.

We can recognise some erratics as being ‘local’ in so much as we can see the rocks in situ at localities on the Yorkshire Coast and further north and then assume those are the sources of the erratics. For example fossils and nodules from the Speeton Clay are quite common as erratics in Holderness and it is tempting to think that they all came from the coastal exposure at Reighton and Speeton. However this is not a large exposure and the erratics are more likely to have come from exposures of the Speeton Clay in the bed of the North Sea when sea level was much lower. Black flints and Belemnitella mucronata occur in Chalk younger than that exposed in East Yorkshire and so must have a North Sea source.

Consistency

How can we compare the data collected by different geologists?

Recently a paper was published about the Quaternary geology at Tunstall (Sutherland et al. 2020). I contacted one of the authors to ask for the definitions of the erratics that were recorded and if I could view them. I was told that definitions of the erratics had not been written and that the specimens illustrated in the paper had been lost.

There are records published in the past by the Boulder Committees but how do we know that the identifications of the erratics are the same as ours? I wonder whether the records of common ‘Augite Syenite’ would now be called Larvikite?

The way forward?

Are there ways in which we can make the recording and collecting less subjective? Perhaps we can adopt some of the methods used for collecting random samples from the diamicts? Bulk sampling and sieving might be taking things a little too far, but can be very useful for quantitative analysis and classification of the diamicts themselves.  However, collecting erratic pebbles from a random square metre of the cliff might force us record and collect all erratics whether we recognise them in the field or not.

Is it not time to create a database of definitions and photographs that is backed up by a reference collection that is in the public domain? The Type Erratic Collection at the University of Hull and associated pages published on the “Ice Age Coast” website is a move in that direction (Horne 2020).

References  

Harmer F W 1928.The distribution of erratics and drift. Proceedings of the Yorkshire Geological Society 21, 79-150.

Horne M 2020. Yorkshire Type Erratics Collection at Hull University and catalogue.  Hull Geological Society web pages

Horne M 2021  Report of the East Riding Boulder Committee 2011 to 2021. Humberside Geologist 16 online.

Sutherland J L, B J Davies, J R Lee 2020. A litho-tectonic event stratigraphy from dynamic Late Devensian ice flow of the North Sea Lobe, Tunstall, east Yorkshire, UK. Proceedings of the Geologists' Association 131, 168-186

 Acknowledgements

I thank Paul Hildreth and Rodger Connell for their comments in the editing stage of this paper.

* Mike Horne BSc PGCHE FGS, Honorary Fellow, Department of Geography, Geology and Environment, University of Hull.

copyright Hull Geological Society 2021